1. Field of the Invention
The present invention relates to a heating apparatus for motor vehicles, and more particularly it pertains to such a heating apparatus wherein engine cooling water is employed as a heat source.
2. Description of the Prior Art
In order to have a better understanding of the present invention, description will first be made of a prior-art heating apparatus for motor vehicles with reference to FIGS. 1 and 2, which includes a casing 1 defining an air flow passage 2. At one end of the air flow passage 2, there are provided ports 4 and 3 which are adapted for taking in air from the vehicle compartment of the motor vehicle and air from outside the motor vehicle, respectively, and at the other end thereof, there are provided air outlet ports 5 which are opened to the compartment. Just for the sake of description, hereinafter, air to be taken in from outside the motor vehicle through the port 3 will be called "outside air", and air to be taken in from the vehicle compartment through the port 4 will be referred to as "inside air". Provided in the air flow passage 2 adjacent to the ports 3 and 4 is an air intake door 6 which is arranged to selectively assume a position to open the port 3 and close the port 4 for permitting outside air to be introduced into the air flow passage 2 through the port 3; a position to close the port 3 and open the port 4 for permitting inside air to be introduced into the air flow passage 2 through the port 4; and a position to open both of the ports 3 and 4 for permitting outside air and inside air to be introduced into the air flow passage 2 through the ports 3 and 4 respectively. In FIG. 2, the port 3 is fully opened while the port 4 is fully closed by the air intake door 6, so that outside air alone is permitted to enter the air flow passage 2 through the port 3. Furthermore, a blower 7 is provided in the air flow passage 2 at a position downstream of the ports 3 and 4, and a heater core 8 through which engine cooling water is passed is also provided therein at a position downstream of the blower 7.
As will be appreciated, with the foregoing conventional apparatus, outside air, inside air or a mixture thereof can be selectively admitted to the air flow passage 2 by operating the blower 7 with the air intake door 6 being set to assume the corresponding one of the aforementioned three positions, and the air thus admitted is passed to be heated to a desired temperature by the heater core. The heated air is blown out into the vehicle compartment.
To rapidly elevate the temperature in the vehicle compartment, the air intake door 6 is set to assume either the position for permitting inside air alone to be admitted to the air flow passage 2 or the position for enabling a mixture of inside air and outside air to be introduced thereinto. By so doing, air warmer than outside air is further heated by the heater core 8 and then blown out into the vehicle compartment from the air outlet ports 5. In this way, rapid elevation of the temperature in the vehicle compartment can be achieved.
However, the above-described prior-art apparatus has the following disadvantage: in such conventional apparatus, air introduced, for circulation, into the passage 2 through the port 4 comprises inside air, i.e., the air in the vehicle compartment which has been blown out thereinto from the heating apparatus, and hence the air thus introduced is at a lower temperature than that of the air just blown out. Thus, with the foregoing apparatus, the temperature of air to be blown out through port 5 can only be elevated to a limited degree. Thus, in case it is desired to rapidly warm up the interior of the vehicle compartment, it is required that the quantity of air to be introduced into the air flow passage 2 be increased to thereby cause the temperature in the vehicle compartment to be rapidly elevated. In such a case, however, another difficulty will be encountered, if it is attempted to meet the aforementioned requirement, in that the humidity of air blown out of the heating apparatus will be increased since the humidity in the vehicle compartment has been increased by the breathing and/or sweating of the passenger or passengers. The increased humidity tends to fog up glass members such as windshield.